1. Field of the Invention
The embodiments discussed herein are related to a semiconductor device and a manufacturing method of a semiconductor device.
2. Description of the Related Art
Conventionally known semiconductor devices include devices that have a package structure in which a semiconductor chip is joined to a circuit pattern that is provided on an insulating substrate, and solder materials that enable joining at a comparatively low temperature are used as joining materials for joining the semiconductor chip and the circuit pattern. As such solder materials, solder having tin (Sn) as a main component, for instance tin-silver (Sn—Ag)-based solder materials and highly reliable tin-antimony (Sn—Sb)-based solder materials that enable joining at a low melting point, is used. The state of a Sn—Ag-based solder material after solder joining and the state of a Sn—Sb-based solder material will be discussed next.
The melting point of a solder having Sn as a main component ranges from about 200° C. to 300° C. A solder joint layer that utilizes solder having Sn as a main component exhibits a structure having Sn crystal grains dispersed therein. In a solder joint layer that utilizes a Sn 100% solder material, Sn crystal grains undergo coarsening at high temperature; moreover, changes in temperature cause the solder joint layer to be subject to stress derived from differences in the coefficient of linear expansion with respect to non-joining materials. As a result, a problem arises in that grain boundary cracks occur at the crystal grain boundaries between Sn crystal grains, and these grain boundary cracks progress to crystal grain boundaries between adjacent Sn crystal grains. Known solder materials in which such progress of grain boundary cracks is prevented include Sn—Ag-based solder materials and Sn—Sb-based solder materials.
FIG. 7 is an explanatory diagram illustrating schematically the state of a solder joint layer by a conventional Sn—Ag-based solder material. In FIG. 7, (a) illustrates an initial state (before application of a thermal load, for instance from power cycling) of a solder joint layer by a conventional Sn—Ag-based solder material (hereafter referred to as Sn—Ag-based solder joint layer). The Sn—Ag-based solder material is a precipitation-strengthened solder material. As illustrated in (a) of FIG. 7, Ag forms virtually no solid solution in Sn crystal grains in solder joint layers that utilize a conventional Sn—Ag-based solder material; accordingly, Ag yields a fine-grained hard Ag3Sn compound 122, and precipitates at crystal grain boundaries between Sn crystal grains 121 that are dispersed as a matrix. The crystal grain boundaries between Sn crystal grains 121 are strengthened as a result and the crystals do not deform readily. Consequently, the grain boundary cracks progress less readily than is the case in solder joint layers of simple Sn crystal grains.
FIG. 8 is an explanatory diagram illustrating schematically the state of a solder joint layer by a conventional Sn—Sb-based solder material. In FIG. 8, (a) illustrates the initial state of a solder joint layer by a conventional Sn—Sb-based solder material (hereafter referred to as Sn—Sb-based solder joint layer).
The Sn—Sb-based solder material is a solder material of solid-solution strengthened type. As illustrated in (a) of FIG. 8, Sb forms a solid solution up to about 8.5 wt % (8.3 atom percent (at %)), such that the Sn crystal grains 131 overall are strengthened, in a solder joint layer that utilizes a conventional Sn—Sb-based solder material in Sn crystal grains 131.
Coarsening of the Sn crystal grains 131 arising on account of thermal load in repeated cycles of heat generation and heat dissipation during the operation of a semiconductor device can be suppressed through strengthening of the Sn crystal grains 131 by the Sb in solid solution. Further, Sb in excess of the solid solution limit precipitates partly in the form of a stiff SnSb compound 132 along with part of the Sn in the Sn crystal grains 131. As a result, the crystals deform less readily, and intra-grain cracks progress less readily.
As such a solder material that includes Sn, Ag and Sb, a solder material has been proposed where, in order to enhance the thermal fatigue characteristic of joints, the content of oxygen (O2) as an unavoidable impurity is set to be equal to or smaller than 5 ppm, and the average grain size to be equal to or smaller than 3 μm, in an Sn alloy solder containing one or two of Ag: 1% to 30% and Sb: 0.5% to 25%, with the balance made up substantially of Sn and unavoidable impurities (see, for instance, Japanese Patent Application Publication No. S61-269998).
As yet another solder material, a solder material has been proposed that includes 5 wt % to 15 wt % of Sb and 2 wt % to 15 wt % of Ag, with the balance made up substantially of Sn, excluding unavoidable impurities, and where the surface roughness of the solder material is Ra=10 μm or smaller (see, for instance, Japanese Patent Application Publication No. H07-284983).
As yet another solder material, a solder material has been proposed that is a composite solder material containing a powder in a solder material, where the solder material includes 5 wt % to 15 wt % of Sb and 2 wt % to 15 wt % of Ag, with the balance made up substantially of Sn, excluding unavoidable impurities (see, for instance, Japanese Patent Application Publication No. H08-001372).
As yet another solder material, a solder material has been proposed that is made up of an alloy including 25 wt % to 40 wt % of Ag, 24 wt % to 43 wt % of Sb, and the balance Sn, where the melting temperature of the solder material is set to at least 250° C. or higher (see, for instance, Japanese Patent Application Publication No. 2003-290975).
As yet another solder material, a solder material has been proposed that includes, in mass %, Ag: 0.9% to 10.0%, Al: 0.01% to 0.50%, Sb: 0.04% to 3.00%, such that the ratio Al/Sb satisfies a relationship of being equal to or smaller than 0.25 (excluding 0), the balance being Sn and unavoidable impurities, where the solder material joins a member having oxide or an oxidized surface (see, for instance, Japanese Patent Application Publication No. 2011-005545).
As yet another solder material, a solder material has been proposed that includes 0.05 mass % to 2.0 mass % of Ag, 1.0 mass % or less of copper (Cu), 3.0 mass % or less of Sb, 2.0 mass % or less of bismuth (Bi), 4.0 mass % or less of indium (In), 0.2 mass % or less of nickel (Ni), 0.1 mass % or less of germanium (Ge), 0.5 mass % or less of cobalt (Co) (where, none of Cu, Sb, Bi, In, Ni, Ge, and Co is 0 mass %), and tin as the balance (see, for instance, Japanese Patent No. 4787384).
As yet another solder material, a solder material has been proposed that has a SnSbAgCu substance as a main component, and the composition of the solder material is 42 wt %<Sb/(Sn+Sb)≤48 wt %, 5 wt %≤Ag<20 wt %, 3 wt %≤Cu<10 wt %, and 5 wt %≤Ag+Cu≤25 wt %, the remainder being unavoidable impurity elements (see, for instance, Japanese Patent No. 4609296).
As yet another solder material, a solder material has been proposed that includes 12 mass % to 16 mass % of Sb, 0.01 mass % to 2 mass % of Ag and 0.1 mass % to 1.5 mass % of Cu, and further includes 0.001 mass % to 0.1 mass % of silicon (Si), and 0.001 mass % to 0.05 mass % of B, with respect to a high-temperature solder material as a whole, the balance being Sn and unavoidable impurities (see, for instance, Japanese Patent No. 4471825).
As yet another solder material, a solder material has been proposed that has, as a main component, a SnSbAgCu substance having a solidus temperature of 225° C., where the constituent ratio of the alloy is 10 wt % to 35 wt % of Ag and Cu, and the weight ratio of Sb/(Sn+Sb) ranges from 0.23 to 0.38 (see, for instance, Japanese Patent Application Publication No. 2005-340268).
As yet another solder material, a solder material has been proposed that has, as a base, an Sn—In—Ag solder alloy including 88 mass % to 98.5 mass % of Sn, 1 mass % to 10 mass % of In, 0.5 mass % to 3.5 mass % of Ag and 0 mass % to 1 mass % of Cu, the Sn—In—Ag solder alloy being doped with a crystallization improver that suppresses growth of an intermetallic phase in the solidified solder (see, for instance, Japanese Translation of PCT Application No. 2010-505625).
As yet another solder material, a solder material has been proposed that includes Ag: 2 mass % to 3 mass %, Cu: 0.3 mass % to 1.5 mass %, Bi: 0.05 mass % to 1.5 mass % and Sb: 0.2 mass % to 1.5 mass %, where the total content of Ag, Cu, Sb and Bi is 5mass% or less, and the balance includes Sn and unavoidable impurities, and the surface properties after reflow are smooth (see, for instance, Japanese Patent Application Publication No. 2002-018590). Reflow is a soldering method where a layer of a solder paste (paste obtained by adding a flux to a solder powder, and adjusted to appropriate viscosity) is formed on a joining material, a component is placed on the solder paste, and thereafter heat is applied to melt the solder.
As yet another solder material, a solder material has been proposed that includes 1 mass % to 3 mass % of Ag, 0.5 mass % to 1.0 mass % of Cu, 0.5 mass % to 3.0 mass % of Bi, 0.5 mass % to 3.0 mass % of In, 0.01 mass % to 0.03 wt % of Ge or 0.01 mass % to 0.1 mass% of selenium (Se), and the balance being Sn (see, for instance, Japanese Patent Application Publication No. 2001-334385).
As yet another solder material, a solder material has been proposed that includes 15.0% to 30.0% of Bi and 1.0% to 3.0% of silver, and depending on the circumstances, optionally 0% to 2.0% of copper, and 0% to 4.0% of Sb and incidental impurities, and the balance being Sn (see, for instance, Japanese Translation of PCT Application No. 2001-520585).
As yet another solder material, a solder material has been proposed that is a Sn—Sb—Ag—Cu quaternary alloy that, with respect to the total, contains proportions of 1.0 wt % to 3.0 wt % of Sb, 1.0 wt % or more but less than 2.0 wt % of Ag, and 1.0 wt % is or less of Cu, with Sn as the balance (see, for instance, Japanese Patent Application Publication No. H11-291083).
As yet another solder material, a solder material has been proposed that contains 3.0 wt % or less of Sb (excluding zero as the range lower limit), 3.5 wt % or less of silver (excluding zero as the range lower limit), 1.0 wt % or less of Ni (excluding zero as the range lower limit), 0.2 wt % or less of phosphorus (P) (excluding zero as the range lower limit), and the balance includes Sn and unavoidable impurities (see, for instance, Japanese Patent No. 3353662).
As yet another solder material, a solder material has been proposed that contains 2.5 wt % to 3.5 wt % of Sb, 1.0 wt % to 3.5 wt % of Ag and 1.0 wt % or less (excluding zero as the range lower limit) of Ni, and the balance includes Sn and unavoidable impurities (see, for instance, Japanese Patent No. 3353640).
As yet another solder material, a solder material has been proposed that includes 0.5 wt % to 3.5 wt % of Ag, 3.0 wt % to 5.0 wt % of Bi, 0.5 wt % to 2.0 wt % of Cu, 0.5 wt % to 2.0 wt % of Sb and the balance being Sn, the solder material being in any one form from among rod-like, wire-like, preform-like or flux-cored solder (see, for instance, Japanese Patent No. 3673021).
As yet another solder material, a solder material has been proposed that includes 0.8 wt % to 5 wt % of Ag, and In and Bi each in an amount of 0.1 wt % or greater, with the total amount of both being 17 wt % or less, and a balance including Sn and unavoidable impurities, and where the solder material has further added thereto 0.1 wt % to 10 wt % of Sb (see, for instance, Japanese Patent Application Publication No. H09-070687).
As yet another solder material, a solder material has been proposed that contains 61 wt % to 69 wt % of Sn, 8 wt % to 11 wt % of Sb, and 23 wt % to 28 wt % of Ag (see, for instance, U.S. Pat. No. 4,170,472).
As yet another solder material, a solder material has been proposed that includes 93 wt % to 98 wt % of Sn, 1.5 wt % to 3.5 wt % of Ag, 0.2 wt % to 2.0 wt % of Cu and 0.2 wt % to 2.0 wt % of Sb, and that has a melting point ranging from 210° C. to 215° C. (see, for instance, U.S. Pat. No. 5,352,407).
As yet another solder material, a solder material has been proposed that includes 90.3 wt % to 99.2 wt % of Sn, 0.5 wt % to 3.5 wt % of Ag, 0.1 wt % to 2.8 wt % of Cu and 0.2 wt % to 2.0 wt % of Sb, and that has a melting point ranging from 210° C. to 216° C. (see, for instance, U.S. Pat. No. 5,405,577).
As yet another solder material, a solder material has been proposed that includes at least 90 wt % of Sn, and an effective amount of Ag and Bi, and that includes optionally Sb, or Sb and Cu (see, for instance, U.S. Pat. No. 5,393,489).
As yet another solder material, a solder material has been proposed that includes 0.5 wt % to 4.0 wt % of Sb, 0.5 wt % to 4.0 wt % of zinc (Zn), 0.5 wt % to 2.0 wt % of Ag and 90.0 wt % to 98.5 wt % of Sn (see, for instance, U.S. Pat. No. 4,670,217).
As yet another solder material, a solder material has been proposed that is a solder paste including a metal component made up of a first metal powder and a second metal powder having a melting point higher than that of the first metal powder, where the first metal is Sn alone, or an alloy including Sn and at least one element selected from the group including Cu, Ni, Ag, gold (Au), Sb, Zn, Bi, In, Ge, Co, manganese (Mn), iron (Fe), chromium (Cr), magnesium (Mg), palladium (Pd), Si, strontium (Sr), tellurium (Te) and P (see, for instance, WO 2011/027659).
Semiconductor devices are subject to thermal load upon repeated heat generation and heat dissipation (power cycling) during operation of the device, and also by thermal load from heat cycles such as changes in environmental temperature or the like (heating and cooling). Deterioration of solder joint layers on account of thermal load derived from such power cycling and the like has conventionally been a problem in semiconductor devices. The life of solder joint layers is one determining factor of the life of semiconductor devices, and it is accordingly necessary to prolong the life of solder joint layers. In order to reduce the size of the semiconductor device as a whole and the size of heat sinks, it is necessary to operate during high-temperature heat generation by the semiconductor device (for instance, 175° C. or higher) and, in particular, to secure the power cycling reliability of power semiconductors at this temperature. Further, semiconductor devices installed in automobiles and semiconductor devices in new energy applications must be long-lived. Consequently, solder materials are required that enable solder joining at a low melting point and that allow forming a solder joint layer of high reliably towards power cycling and the like. Power cycling reliability includes various characteristics of the semiconductor device when the latter is operated and is accordingly subject to a load of a predetermined temperature cycle.
For instance, a Sn3.5Ag solder material (solder material including 96.5 wt % of Sn and 3.5 wt % of Ag) ordinarily used as a conventional Sn—Ag-based solder material described above allows for solder joining at a low melting point (for instance, about 220° C.), but is problematic in terms of reliability during high-temperature operation. When the Ag content in a Sn—Ag-based solder joint layer is increased, as in Japanese Patent Application Publication Nos. S61-269998, H07-284983, H08-001372, 2003-290975, and 2011-005545, material costs increase as well (for instance, a solder cost increase of about 20% for a 1% increase in Ag content), and the melting point becomes higher (for instance, a melting point of about 300° C. in a Sn10Ag solder material (solder material including 90.0 wt % of Sn and 10.0 wt % of Ag)). It is accordingly impractical to increase the Ag content in Sn—Ag-based solder joint layers.
The following problems arise in conventional Sn—Ag-based solder joint layers due to thermal load in power cycling. In FIG. 7, (b) illustrates the state of conventional Sn—Ag-based solder joint layer at the time of a power cycling reliability test (state resulting from being subject to a thermal load from power cycling). In a conventional Sn—Ag-based solder joint layer, Sn crystal grains 121 undergo coarsening on account of thermal load from power cycling, and the Ag3Sn compound 122 undergoes aggregation and coarsening to a grain size of about 5 μm, as illustrated in (b) of FIG. 7.
Consequently, the crystal grain boundaries between Sn crystal grains 121 are no longer strengthened by the Ag3Sn compound 122 and, as a result, a grain boundary crack 123 arises at crystal grain boundaries between Sn crystal grains 121. This grain boundary crack 123 progresses to the crystal grain boundaries between adjacent Sn crystal grains 121.
The above-described conventional Sn—Sb-based solder material is problematic in that although the material becomes more reliable as the content of Sb included in the Sn—Sb-based solder material increases, the melting point of the material rises as the content of Sb increases. For instance, the melting point of a Sn13Sb solder material (solder material including 87.0 wt % of Sn and 13.0 wt % of Sb), which is ordinarily used as a conventional Sn—Sb-based solder material, is of about 300° C. When the semiconductor device is operated in an environment of about 175° C., in some cases even higher reliability may be necessary, depending on, for instance, the intended application of the device, even for Sn—Sb-based solder materials for which the reliability has been enhanced through an increase in the content of Sb to bring the melting point of the solder material to about 300° C.
The following problems arise in conventional Sn—Sb-based solder joint layers due to thermal load in power cycling or the like. In FIG. 8, (b) illustrates the state of a conventional Sn—Sb-based solder joint layer at the time of a power cycling reliability test. In conventional Sn—Sb-based solder joint layers, the crystal grain boundaries between Sn crystal grains 131 are not strengthened, as illustrated in (b) of FIG. 8; accordingly, a problem arises in that, when the solder is strained due to stress, a grain boundary crack 133 forms at crystal grain boundaries between Sn crystal grains 131, and this grain boundary crack 133 progresses to the crystal grain boundaries between adjacent Sn crystal grains 131.
Further, reflow thermal treatments of solder pastes are ordinarily performed in an oven, in a nitrogen (N2) atmosphere. Performing thermal treatment at 300° C. or above is however difficult from the viewpoint of the heat resistance of a solder paste (the heat resistance of the resin in the solder paste is about 250° C.) and thus, solder materials having a melting point of about 300° C. are difficult to use in manufacturing processes. Although thermal treatment is possible at 300° C. or above in reflow thermal treatment of solder paste in an oven having a hydrogen (H2) atmosphere, there is a risk of damage to the semiconductor chip as a result of thermal treatment at a temperature of 350° C. or above. Softening of aluminum (Al) and copper used as electrode materials and structural materials, as well as shorter life and shape defects, are further concerns that arise in a case where the thermal treatment is performed for a prolonged period of about 30 minutes, at a temperature of about 300° C.